My temp gauge fluxuates wildly between 0 and max temp and then sticks on max

Simple test.... Pull wire connector off temp sending unit on drivers side head between the two front spark plugs, ground that wire and turn key on, guage should be maxed out past hot! .. Post test results.

It sounds like you need a new thermometer. Its quite an easy fix and very accessible with these models of chevys. Not very expensive either. Installation is simple and i recommend looking at a video.